1. Field of the Invention
The present invention relates to a brake pressure control device for vehicles for controlling the brake pressure to provide, e.g. an antilocking function, a traction control, an automatic brake control for a vehicle-to-vehicle distance control, an automatic brake control for an obstacle avoidance, etc.
2. Description of the Prior Art
As the simplest method of controlling brake pressure for antilocking control, traction control, automatic brake control for vehicle-to-vehicle distance control, automatic brake control for obstacle avoidance etc., it is well known to provide an electromagnetic directional control valve, which is switched in a plurality of positions by an electromagnetic force, in a pipeline connecting a master cylinder and a wheel brake. A typical brake pressure control device for an antilocking control is disclosed in U.S. Pat. No. 3,617,098, for example. Present FIG. 1 is a hydraulic pressure circuit diagram of the brake pressure control device as disclosed in U.S. Pat. No. 3,617,098.
Referring to FIG. 1, an electromagnetic directional control valve 4, which is switched between two positions by an electromagnetic force, arranged in a pipeline connecting a master cylinder 2 for converting an operating force of a brake pedal 1 into a hydraulic pressure for a wheel brake 3. An auxiliary power circuit 5 is connected in parallel with the electromagnetic directional control valve 4. A pump 7 is arranged in the auxiliary power circuit 5, to be driven by a motor 6. An electromagnetic directional control valve 8, which is switched between two positions by an electromagnetic force, and a reservoir 9 are further arranged in the auxiliary power circuit 5, as shown in FIG. 1. A check valve 10 is connected in a circuit 11 which is arranged in parallel with the electromagnetic directional control valve 4. The check valve 10 allows flow of fluid from the wheel brake 3 to the master cylinder 2, while preventing reverse flow of the fluid.
It is assumed here that an antilocking signal is generated when the brake pedal 1 is operated in the hydraulic pressure circuit as shown in FIG. 1. The positions of the electromagnetic directional control valves 4 and 8 are suitably switched by the signal. Pressure against the wheel brake 3 is reduced when both of the electromagnetic directional control valves 4 and 8 are energized and retained when only the electromagnetic directional control valve 4 is energized, while being increased when the electromagnetic directional control valves 4 and 8 are not energized. Under such antilocking control, the pressure against the wheel brake 3 is controlled to be lower than that of the master cylinder 2. However, when the driver relaxes the operating force against the brake pedal 1 for relaxing the braking force during the control, the brake pressure must be lowered responsively. The check valve 10 is provided in order to satisfy such requirement. If, for example, the operating force against the brake pedal 1 is relaxed when the electromagnetic directional control valve 4 is energized, the pressure of the master cylinder 2 becomes lower than that against the wheel brake 3, whereby the fluid passes through the check valve 10 to flow toward the master cylinder 2. Thus, the brake pressure against the wheel brake 3 is also lowered in response to relaxation of the operating force by the brake pedal 1.
In case where the brake pedal 1 is not operated in an automatic brake control for a traction control or vehicle-to-vehicle distance control, for example, the pressure against the wheel brake 3 must be increased when the driver should operate the brake pedal 1 the pressure against the wheel brake 3 is higher than the automatic control pressure. Such an operation is required in a quick braking action of the beginning of a traction control, for example. In order to satisfy such a requirement, it is necessary to provide another check valve, which is effective in a direction opposite to that of the check valve 10, in the hydraulic pressure circuit as shown in FIG. 1. The newly provided check valve is adapted to allow flow of the fluid from the master cylinder 2 to the wheel brake 3 while preventing a reverse flow. When the brake pedal 1 is operated in such circuit structure, the pressure against the wheel brake 3 is also increased responsively.
A device not employing any check valve is also proposed in the art as a system for varying the pressure against a wheel brake with a change in the operating force of a brake pedal during an antilocking control or a traction control. In U.S. Pat. No. 4,462,642, hydraulic pressure sensors are arranged in front and at the back of a cutoff valve, which is connected in a pipeline connecting a master cylinder and a wheel brake, to compare hydraulic pressure levels on both sides of the cutoff valve with each other whereby the operation of the cutoff valve is controlled in response to the hydraulic pressure levels.
In the brake pressure control device as shown in FIG. 1, the check valve 10 must be connected in parallel with the electromagnetic directional control valve 4. Thus, the circuit structure and pipe arrangement are complicated, leading to economic loss and reduced reliability.
Japanese Patent Laying-Open Gazette No. 202142/1983 also discloses a brake pressure control device having a check valve connected in parallel with an electromagnetic directional control valve. This brake pressure control device is also complicated in its circuit structure, similarly to the brake pressure control device of U.S. Pat. No. 3,617,098 as shown in FIG. 1.
In particular, a rather complicated circuit structure is required in order to vary the pressure against the wheel brake in response to a change in the operating force of the brake pedal when an antilocking control is performed while the brake pedal is being operated and when a traction control performed while the brake pedal is not being operated.
Further, when hydraulic pressure sensors are provided on both sides of a cutoff valve as in the brake pressure control device disclosed in U.S. Pat. No. 4,462,642, the total cost of the entire device is increased since the hydraulic sensors etc. are expensive.